Change Auth::guest() to call the check() method.

Instead of reimplementing the logic of the Auth::check() method, we can call and negate it. This makes it easier to change things in a Guard subclass.
Merge pull request #2439 from franzliedke/patch-12

Change Auth::guest() to call the check() method.
Fix unguarded bug.
Merge branch '4.0' of https://github.com/laravel/framework into service-providers-4.0